Air disconnect switch



May 1, 1956 H. P. SCHNEIDER 2,744,179

AIR DISCONNECT SWITCH 3 Sheets-Sheet 1 Filed Feb. 2, 1953 INVENTOR.

Hans .,/;S hn%e r y 1956 H. P. SCHNEIDER 2,744,179

AIR DISCONNECT SWITCH Filed Feb. 2, 1955 3 Sheets-Sheet 2 IN VEN TOR.

y 1, 1956 H. P. SCHNEIDER 2,744,179

AIR DISCONNECT SWITCH Filed Feb. 2, 1953 3 Sheets-Sheet 5 INVENTOR. Scllnei er United States Patent AIR DISCONNECT SWITCH Hans P. Schneider, Milwaukee, Wis., assignor to McGraw Electric Company, Milwaukee, Wis., a corporation of Delaware Application February 2, 1953, Serial N 0. 334,630

9 Claims. (Cl. 200-146) This invention relates generally to air disconnecting switches and in particular to electrical switches that are adapted to be opened under load.

It is often necessary to open a high voltage disconnecting switch while normal load current is flowing in the circuit where the switch is installed. To do so involves interrupting the current, which is usually done by separating one end of the switch blade from its cooperating terminal. As the switch blade is opened, arcing occurs which burns or fuses the contacts. Repeated reopening of the switch with repeated arcing will eventually destroy the contacts. There is also the danger under severe current conditions that a long are may be sustained and establish itself between the switch and another circuit or ground. It is to avoid this dangerous and destructive arcing condition that this novel switching device was developed.

An object of this invention is to interrupt the current of an air interrupter switch in an arc extinguishing enclosure removed from the contact terminals.

Another object of this invention is to provide a means of continuing the current flow while one end of the switch blade is separated from one terminal and until said end of switch blade is sufficiently spaced from said terminal that the current can be extinguished without danger of flash over.

A further object of this invention is to provide a cocking device synchronized with the opening movement that returns the arc extinguishing means to operable position after the arc extinguishing means has functioned.

And another object of this invention is to provide a locking device whereby when the switch is being opened, it cannot be reclosed until the opening operation is completed and whereby when the switch is being closed it cannot be reopened until the closing operation is completed.

Other objects will appear from time to time in the course of the specification and claims.

This invention is illustrated in the accompanying drawings in which:

Fig. l is a side elevation showing the switch blade in closed position, the intermediate and open position shown in phantom outline.

Fig. 2 is a sectional view of the switch blade in closed position.

Fig. 3 is a sectional view of the switch blade in intermediate position.

Fig. 4 is a sectional view of the switch blade in open position.

Fig. 5 is an enlarged top view of the locking device as the blade is being closed and taken approximately 0n the line 5-5 of Fig. 1.

Fig. 6 is the same as Fig. 5 illustrating the locking device as the blade is being opened.

Figs. 7, 8, and 9 illustrate the action of the latch and latch arm.

Fig. 10 is a fragmentary sectional view of the locking device taken approximately on the line 10--10 of Fig. 6.

Referring to Fig. 1, the reference character 1 designates a base on which suitable insulators 2 and 3 can be mounted. The base 1 may be in the form of a metallic channel and though it is shown in a horizontal position, it could be mounted vertically or diagonally or even hori- "ice 2 zontally but upside down to the position illustrated in Fig. l.

The insulators 2 and 3 support the stationary switch members 4 and 5 which have extensions 6 and 7 to which line terminals can be attached. For interconnecting the stationary switch members 4 and 5, a switch blade shown generally at 8, is pivotally mounted at pivot point 9 in a hinge housing 15 on the switch member 4 and is arranged to be moved into and out of engagement with the switch member 5.

The switch blade 8, pivotally mounted at 9, rests at its opposite end in a contact jaw 10 mounted on the terminal 5. Also attached to the terminal 5 and extending upwardly is a stationary auxiliary contact 11 which carries the current while the switch blade is being pivoted upwardly out of the contact jaw 10. When the blade is in position B, shown in phantom outline, current has been flowing through the stationary auxiliary contact 11 and is being broken within the switch blade housing. After the current is broken, the blade can be moved to position C out of contact with the stationary auxiliary contact 11 and while being moved to this vertical position, the arc extinguishing mechanism is recocked by the movement of a cocking arm 12 traveling in a groove 13.

The cocking arm 12 has its ends turned at right angles in opposite directions. One bent end pivots at 14 in the hinge casting 15 while the other end of the cocking arm 46 rides in the slot 13 which extends along the side of the arc extinguishing mechanism within the switch blade.

A pull ring 16 mounted on the switch blade 8 has a latch operating lever 17 extending downwardly with a positive latch under pressure to hold the blade in closed position until the latch is opened. This latching device is not new and so it will not be described or illustrated in detail here.

Figs. 2, 3, and 4 illustrate the action that takes place within the switch blade 8. The blade pivoting at pivot point 9 on the hinge casting 15 comprises an outer conductive housing 18 which normally carries the current between the two terminals. A cap 19 at the pivot end is conductive while a cap 20 at the outer end of the blade is non-conductive.

Within the blade housing 18 is a tubular member 21 made of silicon bronze or some other conductive metal of high mechanical strength. This tubular member 21 is held by the cap 19 at the pivot end and at its other end fits into an internal annular shoulder 22. The internal annular shoulder 22 also functions as an anchor for a Lucite tubular member 23 which is the lining for the outer half of the switch blade. The inner surfaces of the Lucite tubular member 23 and the shoulder 22 are in line with each other. The Lucite tube being a non-conductor is the arc extinguishing chamber of the switch blade.

The switch blade comprises two conductive paths, the normal one along the outer housing and an auxiliary path within the blade that functions while the blade is being opened to the position of Fig. 3. The auxiliary path extends along the internal mechanism which consists of a metal rod 24 that slides in and out of a follower rod assembly designated generally as 25. The follower rod assembly 25 is a combination of conductive and nonconductive materials. It has a metal tubular portion 26 which slides over the conductor rod 24 and a metal circular stop 27 which serves to stop the movement to the right of the follower rod assembly when it strikes against the internal annular shoulder 22. To the right of the stop 27 is a fibre tubular member 28 with a brass contact head 29 on its outer end. To connect the follower rod assembly 25 conductively to the outer conductor terminal assembly 30 of the blade is a flexible cable 31. The flexible cable 31 is attached to an extension rod 32 of the brass contact head 29 and a similar extension rod 33 of the terminal assembly 30.

The auxiliary terminal assembly 30 comprises a pair of leaf members 34 which extend on each side of the auxiliary contact 11 to guide the switch blade 8 so that contact is assured between the blade and contact 11 as the blade ismoved-a'wayfrom "the contact jaw 10 and the contact tongue 35 rides along the upward extending auxiliary contact 11-.

. A latch arm 36 pivoting at37 in the lower protrusion 18:: of the housing '18 has a tongue 38 extending through a slot 39in the annular shoulder '22. The latch'arm 36 is held upwardly by pressure fro'rn'aspring 40 actuating a bolt head 41 so that the tongue 38 presses against the brasscontact head 29 and holds the follower rod-assembly 25 in the position shown in Fig. 2. At the opposite end of the latch arm from the'tongue-3S is a rod 42 over which a latchmember 43rests. The latch member 43 pivots at 4.4 on the hinge casting 1'5 and as the switch blade is swung upwardly on pivot to approximately l8-, the latch arm 36is carried out from under the latch member 43 to the .position shown in Fig. 3. As the latch arm 36 .pivots downwardly on .pivot :point 37, the tongue 38 also swings downwardly and releases the follower rod assembly 25 within the switch blade.

The-follower rod assembly 25 under pressure from a spring 45 moves awayfrom the rod 24 to the position shown in Fig. 3; This places the fibre tubular member 28 within the Lucite tubularliner-23 and as both are nonconductors and have arcquenchingproperties, the current is broken and the arc quenched in this area.

.-Fig. 4 is a sectional view of the C position of the blade in Fig. 1. When the blade 8 is moved to this vertical position the cocking arm-12 with its inwardly turned end 46 riding in the groove 13 catches the metal circular stop 27 of the follower rod-assembly and pulls the follower rod assembly 25 downwardly-in the blade -8 to the position shown in Fig. 4. This allows the latch 36 to spring back so that the tongue 38 again holds the follower rod assembly in cocked position within the blade. This cocked position is retained while the blade is returned to closed position and continues to be maintained within the blade until the next timethe blade is opened.

The'latch action'between the latch arm 36 and latch member 43 is shown in Figs. 7, '8, and 9. As the switch blade 8 is being closed the rod 42 of the latch arm 36 rides along the top of latch member '43 as shownin Fig. 7. This movement pushes aside the latch member 43 as shown in Fig. 8 until the rod 4201": the-latch 'a'rm36 slips under the'head of the latch member 43 and is held there byiit asfshown in'Fig. 9. During theopening movement of the switchblade 8 the head of the latch member 43 holds the'rod '42, forcing that end of the-latch arm to swingdownwardly againstthe spring 40 as the pivot 37 movesupwardly with theswitch blade. As the upward swing of the blade 8 continuesjthe rod 42moves along the under surfaceof the bead'of the latch-member 43 until'it is free. Figs. 2iand 3 show how this would occur. In-Figs. and'6, a cam'47'is shown attached to the right hand hinge housing 15 .by an arm 48. The end of the rod 42 opposite the latch 43 rides along the side of the cam 47 after the rod 42 is free fromthe latch 43. To illustrate the relationship of the cam 47 to the latch and rod'action the cam is shown in phantom line in Figs. 7, 8' and 9. This cam 47 takes over and by means of rod 42 guides the movement of the latch arm '36 during the opening swing of the-blade so that the end of the latch arm near the rod 42 is pushed downwardly moving the tongue 38 away fromthe position where it retains the follower'rod assembly 25. During the further upward swing'ofthe'blade'fi, the rod 42 circles around the cam 47 until in the open position C it is above the carn 47 A locking'device to control the opening and closing movements of the blade so that the'bla'des cannot be closed before it is recocked or "reopened before it has been completely closed, is located in the hinge'housing' 1 5,'and a-pair of leg'members 50 which are attached to in the inward position.

4 the blade 8. This can be seen in Figs. 2, 4, 5, and 6. The legs 50 extending diagonally downward on each side of the blade 8 are in fixed relationship with the blade, rotating with the blade as it pivots on the pin 9. At the lower end of the legs 50, at the opposite end from the pivot 9 is a connecting tubular housing 51 which holds two slidable pawl members 52 and 53 with a tension spring 54 between them. The inner surfaces of the hinge housing l 15 have circular tracks with graduated surfaces to coincide with the rotating route of the tubular housing 51 and to govern a tripping action of the two pawls 52 and 53.

Fig. 5 illustrates the blade closingmovement and Fig. 6 the blade opening movement. The legs 50 are cut away in both views so that the action of the pawl members 52 and 53 along the graduated tracks of the hinge housing 15 can be seen more easily.

The pawl members 52 and 53 each 'have an annular notch 61 which serves as a means of holding each pawl The notch '61 can be seen in Fig. 6, and in Fig. 10 it can be seen how the pawl is held by a metal dog 62, urged by a spring 63, locating in the notch and holding the pawl until a persistent thrust forces the metal dog 62 upwardly against the spring pressure 63. This thrust is actuated by the graduated track in the opposite hinge housing 15 pushing the other pawl inwardly to the point where it forces outwardly the pawl held by the metal dog. When the graduated track forces a pawl inwardly'to where its notch 61 is in line with'its corresponding metal dog, the metal dog will drop intothe notch and hold that pawl.

The track that operates during the closing operation is shown-on the fragment of the hinge housing of Fig. 4 as wellas inFig. 5. The top protuberance S5 in cooperation with the-pawl member 52 serves as a stop to prevent further opening of the switch blade beyond the position shown in Fig. 4. As the switch blade 8 is moved downwardly, a graduated track 56 governs the movement of the pawl 52 inwardly in the housing 51. This can be seen in Fig. 5. When the switch bladehas pivoted downwardly to the point where it makes contact with the auxiliary contact horn '11, the pawl member 52 has traversed the graduated track 56 and slipped behind its highest edge at '57. This prevents reopening of the switch blade after a circuit has been established during the downward movement. The last graduated elevation 58, pushes the pawl member 52 inwardly a sufiicient distance to trip the other pawl member 53 outwardly on the opposite side so that pawl member 53'is'in position fora subsequent opening of the switch blade.

The opening movement of the switch blade 8 putsinto action a' locking device between the pawl member 53 and the right hand hinge housing 15 of Fig.6. When the switch blade is closed, the lower portion of the leg 50 is in the position shown in Fig. 2 and the tubular housing 51 holdingthe pawl "members 52' and 53 is beyond the first or'lowest protuberance 59 on theright-hand hinge housing 15 of Fig. 6. The protuberance 59 develops angularly to correspond with the beginning of the opening movement of the switch blade so that by the time the blade is disconnected from the contact jaw-10, the pawl member 53 has slipped within the confining edge of the protuberance -59 as shown in Fig. 6. This prevents'the possibility of closing the blade after the circuit interrupting process has'begun, or before the opening movement'of'the blade has been completed whereby the circuit "interrupter is returned to its cocked position.

With further opening-movement of-the switch blade 8, the pawl 53 travels along'a'graduatedtrack 60,'moving inwardly in the'tubular housing 51 as the track 60 becomes wider. This inward movement of the pawl 53 trips the pawl member 52in the tubular housing '51 so that the pawl '52"movesoutwardly from the other end of the tubular-housing 51. When the pawl 52 is in this outward position it strikes against the protuberance'which serves as a stop to prevent further opening of the switch'blade.

When the pawl 52 is tripped to this outward position it signals that the circuit interrupter has been returned to cocked position and that therefore the blade is in condition to be closed. If the opening movement is not completed, the pawl 52 will not be tripped by the inward thrust of the pawl 53 so that the pawl 53 urged by spring pressure moves outwardly again and strikes against the stop side of the protuberance 59 on the downward movement.

Thus, it can be seen that with this locking device, any possibility of closing the switch blade before recocking the circuit interrupter is eliminated. Also, the danger of reopening the switch blade after the auxiliary circuit has been established on the downward movement is prevented. These safety measures assure an air interrupter switch that is automatically recocked and in condition to be operated and that cannot he accidentally reopened at a dangerous point.

The use and operation of my invention are as follows:

The purpose of this air interrupter switch is to provide in electrical distribution or transmission lines a place where an air gap can be established during normal load. It is not to be used when there is greater than normal load.

The switch blade when closed carries the circuit along its outer housing between the two terminals. As the blade is opened an auxiliary circuit through the interior of the blade and the auxiliary contact horn is established. With two circuits existing between the terminals one circuit can be broken without causing a flashover or arcing. The principal contact between the contact jaws and the housing of the switch blade 8 can be separated safely and the auxiliary circuit maintained until the switch blade is a safe distance from the contact jaws.

At this safe point in the upward movement of the switch blade, the pivoting movement of the latch arm 36 due to the holding action of the latch member 43, releases the circuit interrupter within the switch blade. This action is shown in Fig. 3. The tongue 38 has moved away from the brass contact head 29, allowing the spring 45 to drive the follower rod assembly into circuit interrupting position. The fibre tube 28 is juxtaposed to the Lucite liner tube 23 and since both tubes are non-conductors, the circuit is broken.

The blade opening movement is continued after the circuit is interrupted. After the contact assembly of the blade disconnects with the auxiliary contact 11 the cocking arm 12 returns the follower rod assembly 25 to its original position to reestablish the auxiliary conductive patch through the interior of the switch blade. The action of the cocking arm is illustrated in Fig. 1 by the relative positions of the cocking arm in positions A, B, and C. In Figs. 3 and 4, the inwardly turned end 46, in section, shows how the cocking arm catches the metal stop 27 and draws it downwardly carrying the follower rod assembly with it. When the metal tubular portion 26 slides over the metal rod 24, the auxiliary conductive path from the brass conductor head 29 through the metal tubular enclosure member 21, to the stop 27, the tube 26, the rod 24, and the cap 19, is established to the terminal 4.

As explained previously, the locking device in the hinge housing 15 and legs 59 makes it necessary to open the blade to the position of Fig. 4 with consequent recocking action before the blade can be closed.

The latch arm 36 and rod 42 after circling the latch member 43 and cam 47 and freeing itself thereby, pivots to the position shown in Fig. 4 under pressure from spring 40. The tongue 33 is back in position where it will hold the brass conductor head 29 when the downward movement of the blade and consequent cocking arm movement allows the follower rod assembly to return to the position of Fig. 2.

During the downward movement of the blade 8 the auxiliary circuit is established when the contact assembly 30 touches the auxiliary contact horn 11. Thus, when the switch blade 8 reaches the contact jaws 10 there is no arcing or flashover resulting from closing the switch. The locking device prevents any upward movement of the blade after the auxiliary circuit has been established.

Thus, it can be seen that with this air interrupter switch for high voltage lines, the switch can be opened or closed under normal load without any arcing or fiashover that would be dangerous to the lineman and destructive to the principal contact points. A switch like this contributes safety and economy and therefore fills a great need.

1 claim:

1. An electrical switch comprising a contact structure, a hinge contact, a longitudinally slotted switch blade supported on said hinge contact for movement into and out of contact engagement with respect to said contact structure, an auxiliary conductive path through the interior of said switch blade, an auxiliary contact horn extending from said contact structure that engages said switch blade as it is being moved into and out of contact engagement with said contact structure, said auxiliary contact horn and said auxiliary conductive path through the interior of said switch blade forming a closed circuit before said switch blade is moved out of contact engagement with said contact structure and maintaining said closed circuit after said switch blade is out of contact engagement with said contact structure, said auxiliary conductive path through the interior of said switch blade comprising a follower rod mechanism releasably held against spring pressure, said follower rod mechanism when released, breaking said auxiliary conductive path within said switch blade, a control arm actuated by movement of said switch blade which when said switch blade is moved to open position returns said follower rod mechanism from released position to unreieased position, said control arm having one end pivoted on said hinge contact and another end engageable with said follower within said switch blade through said longitudinal slots.

2. An electrical switch comprising a contact structure, a switch blade supported for movement into and out of contact engagement with respect to said contact structure, an auxiliary conductive path through the interior of said switch blade, an auxiliary contact horn extending from said contact structure that engages said switch blade as it is being moved out of contact engagement with said contact structure, said auxiliary contact horn and said auxiliary conductive path through the interior of said switch blade forming a closed circuit before said switch lade is moved out of contact engagement with said contact structure and maintaining said closed circuit after said switch blade is out of contact engagement with said contact structure, said auxiliary conductive path through the interior of said switch blade comprising a follower rod mechanism releasably held against spring pressure, said follower rod mechanism when released, breaking said auxiliary conductive path within said switch blade, a control arm actuated by movement of said switch blade which when said switch blade is moved to open position returns said follower rod mechanism from released position to unreleased position, a locking means operating between said switch blade and said contact structure during opening and closing, to prevent closing the said switch blade before said follower rod mechanism has been returned to circuit making position in the auxiliary conductive path and to prevent opening movement of said switch blade before closing movement has been completed.

3. An electrical loadbreak switch comprising in combination a disconnect switch having a hinge contact and a swinging blade pivoted thereon, a contact engageable and disengageable by the blade, said blade being adapted to move through a series of angular positions in its opening and closing movements; an auxiliary contact horn attached to said contact and engageable and disengageable by said blade during opening and closing movements, an auxiliary conductive path through said blade to make a closed circuit with said auxiliary contact horn; a cir- 7 cuit interrupter in said auxiliary conductive path releasably held against spring pressure, a latch means operated by the opening movement of said swinging blade to release said circuit interrupter; a control arm operated by the complete opening movement of said swinging blade, to return the released circuit interrupter to circuit closing position in the auxiliary conductive path, a locking means including a pair of relatively spaced arcuate tracks in parallel relation adjacent the pivot end of said blade, coaxial pawls carried by the switch blade and each biased toward engagement with an adjacent track, means on said blade for alternately locking either of said pawls in engagement with its corresponding track, one of said tracks having a pawl catch engageable therewith to prevent swinging of said blade in one direction and a pawl looking protuberance engageable when the blade is swung in the opposite direction to lock said pawl and release the other of said pawls for engagement with the other of said tracks, the other of said tracks having a pawl catch and a pawl locking protuberance operable respectively when the blade is swung in the opposite of said directions.

4. An electrical loadbreak switch comprising in combination a disconnect switch having a hinge contact, a swinging blade pivoted thereto, a contact engageable and disengageable by the blade, said blade being adapted to move through a series of angular positions in its opening and closing movements; an auxiliary contact horn attached to said contact and engageable and disengageable by said blade during opening and closing movements, an auxiliary conductive path through said blade to make a closed circuit with said auxiliary contact horn; a circuit interrupter in the said auxiliary conductive path releasably held against spring pressure, a latch means operated by the opening movement of said swinging blade to release said circuit interrupter; a control arm operated by the complete opening movement of said swinging blade, to return the released circuit interrupter to circuit closing position in the auxiliary conductive path, a locking means in combination with the hinge contact to assure an opening swing of the blade to where the auxiliary conductive path is reclosed, and to assure the completion of the closing swing of the blade after closing has been begun.

5. A switch adapted to be opened under load comprising two terminal members, switch means for connecting and disconnecting said terminal members by movement to closed and open position respectively, means for moving said switch means, an auxiliary contact horn extending from said terminal where the switch means is opened and an auxiliary conductive path within said switch means in contact with said contact horn to form thereby a second circuit between said terminal members while said switch means is being disconnected, a circuit interrupter in the said auxiliary conductive path releasable to circuit interrupting position by movement of said switch from closed to open position, said releasable means operating after the circuit through said auxiliary conductive path and auxiliary contact horn has been actuated, a cocking arm to return said circuit interrupter to closed circuit position in said auxiliary conductive path after said circuit interrupter has been released, locking means operating between the closed end of said switch and its corresponding terminal to assure complete opening and recocking of said circuit interrupter before said switch can be closed and to assure complete closing of said switch before subsequent reopening.

6. A switch adapted to be opened under load comprising two terminal members, a switch blade pivoted to one of said terminals, means for connecting and disconnecting the said switch blade from the other of said terminals by movement to closed and open position respectively, means for moving said switch blade, an auxiliary contact horn extending from said terminal where said switch blade is opened, an auxiliary conductive path within said switch blade which contacts said contact horn while said switch blade is being moved to open position and establishes an auxiliary circuit therethrough, a circuit interrupter in the said auxiliary conductive path releasable to circuit interrupting position by movement of said switch blade after establishing said auxiliary circuit, a latch arm operated by the opening movement of said switch blade to release said circuit interrupter, latch hook means for engaging said latch arm and maintaining it in unreleased position, a cam member afiixed on said one terminal for guiding said latch arm into and out of engagement with said latch means, a cocking arm actuated by relative movement between said Switchblade and said pivotally connected terminal member, said movement of said cocking arm returning said releasable circuit interrupter to circuit making position in the said auxiliary conductive path within said switch blade as said switch blade is moved 'toopen position.

7. A switch adapted to be opened under load comprising two terminal members, a switch blade pivoted to one of said terminals, means for connecting and disconnecting the said switch blade from the other of said terminals by movement to closed and open position respec tively, means for moving said switch blade, an auxiliary contact horn extending from said terminal where said switch blade is opened, an auxiliary conductive path within said switch blade which contacts said contact horn while said switch blade is being moved to open position and establishes an auxiliary circuit therethrough, a circuit interrupter in the said auxiliary conductive path releasable to circuit interrupting position by movement of said switch blade after establishing said auxiliary circuit, a cocking arm actuated by relative movement between said switch blade and said pivotally connected terminal member, said movement of said cocking arm returning said releasable circuit interrupter to circuit making position in the said auxiliary conductive path within said Switchblade as said switch blade is moved to open position, locking means operating at the pivot end of said switch blade to assure complete opening of said switch blade, with its recocking action, and complete closing of said switch blade before subsequent reopening.

8. A locking means in combination with a resetting load interrupter and a pivotally mounted switch blade adapted to swing in and out of contact engagement with a fixed switch terminal, said locking means including laterally spaced graduated tracks in opposed relationship relative to one another adjacent the pivot end of said switch blade, pawl means positioned transversely relative to said tracks and alternately engageable and disengageable with said tracks respectively during swinging movement of said blade, whereby said switch blade is prevented from closing unless swung to a predetermined open position and is prevented from opening unless swung to a predetermined closed position.

9. A locking means in combination with a resetting load interrupter and a pivotally mounted switch blade adapted to swing in and out of contact engagement with a fixed switch terminal, said locking means including complementary graduated arcuate tracks laterally spaced adjacent the pivot end of said switch blade, coaxial pawls carried by said switch blade and swingable therewith between said graduated tracks, a compression spring between said pawls alternately biasing said pawls into and out of engagement with said graduated tracks, whereby said switch blade is prevented from closing unless swung to a predetermined open position and is prevented from opening unless swung to a predetermined ciosed position.

References Cited in the file of .thispatent UNITED STATES PATENTS 2,239,224 Goldner Apr. 22,1941 2,272,717 *Ludwig et al Feb, 10, :1942 2,337,889 Harlow er al Dec/'28, [1943 2,344,113 Schultz W Mar. 14, .1944 2,663,779 Viboud et al Dec. 22, 1953 

